1. Field of the Invention
The present invention relates to a brake control system for an electrically operated vehicle to perform a regenerative braking operation and a hydraulic braking operation, and more particularly to the brake control system which is provided with a static pressure generator and a dynamic pressure generator, and which is adapted to apply a braking force to a wheel by supplying a dynamic hydraulic pressure from the dynamic pressure generator in a transitional period from the regenerative braking operation to the hydraulic braking operation.
2. Description of the Related Arts
In an electrically operated vehicle provided with an electric motor as a power source, the electric motor also functions as a generator for performing the regenerative braking operation to recover energy by charging a battery and increase the energy when driving the motor. Since the braking force applied to the vehicle wheels through the regenerative braking operation is limited, it must be compensated by a hydraulic braking operation. Therefore, both of the regenerative braking operation and the hydraulic braking operation are performed, in general. In order to add the hydraulic braking operation to the regenerative braking operation, for example, a relief valve is disposed between a master cylinder and a wheel brake cylinder to be opened when a difference in pressure between the master cylinder and the wheel brake cylinder exceeds a predetermined value.
In order to increase the efficiency of recovering the energy through the regenerative braking operation, various apparatuses have been proposed. According to a brake apparatus disclosed in Japanese Patent Laid-open Publication No.7-336806 for example, an ON/OFF valve is disposed between the master cylinder and the wheel brake cylinder, and closed when the regenerative braking operation is performed. In the Publication, it was raised as an issue to be solved that a brake pedal was moved forward to vary the depressing force, when the ON/OFF valve was switched from its closed condition to its open condition so as to cancel a mode for giving a priority to the regenerative braking operation. Therefore, the brake apparatus was proposed to rapidly increase the pressure in a hydraulic braking device by temporarily introducing a hydraulic pressure from a hydraulic booster.
With respect to the braking force applied through the regenerative braking operation as described above, FIG. 9 illustrates its limit (Fu). When a vehicle speed is in an intermediate speed zone (Vm), the limit (Fu) is provided for the braking force applied through the regenerative braking operation, due to mainly mechanical factors such as output torque. When the vehicle speed is in a high speed zone (Vh), the braking force applied through the regenerative braking operation is lowered in accordance with the increase of vehicle speed, due to mainly electrical factors such as an inverter, battery or the like. And, when the vehicle speed is in an extremely low speed zone (Vs), it is preferable not to perform the regenerative braking operation, because the energy consumed through the generating operation by the motor will be more than the energy recovered through the regenerative braking operation, and a vibration might occur during the regenerative braking operation. Therefore, when the braking force is applied to the vehicle moving at a speed "V1" as shown in FIG. 10, for example, it is preferable to apply the braking force only by the regenerative braking operation until the time "t1" as shown in FIG. 10. After the time "t1", the hydraulic braking operation may be added to the regenerative braking operation to apply the braking force by both of the regenerative braking operation and the hydraulic braking operation, and in the extremely low speed zone after the time "t2", only the hydraulic braking operation should be performed.
In such conditions as described above, when the braking operation is switched from the regenerative braking operation to the hydraulic braking operation, the master cylinder pressure is temporarily reduced until the wheel cylinder pressure is increased close to the master cylinder pressure, so that the stroke of the brake pedal is rapidly varied and the vibration of the depressing force occurs to deteriorate the brake pedal feeling, as described in the Publication No.7-336806. According to the apparatus proposed in the Publication, when the difference of the pressure between relief valves (VR1), (VR2) is to be cleared, a solenoid valve (V1) is closed at the outset and subsequently a solenoid valve (V4) is opened to temporarily introduce the hydraulic pressure from a hydraulic booster (HB), so as to prevent a large pressure difference from being caused between the master cylinder pressure and the wheel cylinder pressure.
According to the apparatus proposed in the Publication, however, the hydraulic booster (HB) included in a dynamic pressure circuit is connected to a static pressure circuit between the master cylinder and the wheel brake cylinder through the solenoid valve (V4). The static pressure circuit includes the relief valve (VR2) for use in adding the hydraulic braking operation to the regenerative braking operation or changing for it, a solenoid valve (V5) and a check valve (VC2). Therefore, when the hydraulic braking operation is changed for the regenerative braking operation, brake fluid is supplied from the hydraulic booster (HB) to the static pressure circuit, so that more amount of brake fluid is supplied from the hydraulic booster (HB) to the wheel brake cylinder, than the brake fluid supplied from the master cylinder. As a result, sealing members in the master cylinder might be deteriorated due to the residual pressure therein. Furthermore, in the apparatus according to the Publication, two pressure circuits are connected to each other through the solenoid valve (V4). Therefore, if malfunction occurs in the static pressure circuit with the solenoid valve (V4) held in its open position, the brake fluid might be discharged through the dynamic pressure circuit.